If you can do ray tracing fast, you can do rasterization faster. And so if the ray tracing is not critical, you're wasting time computing it.
Besides, computational power isn't something we necessarily need to conserve for its own sake. We stopped using Gourad shading when the hardware became fast enough that we didn't need it anymore. Polygon rasterization will stick around for a long time I think, but I don't think it will always be an essential part of everyone's real-time rendering technology stack.
http://www.paulgraham.com/hundred.html
> I can already tell you what's going to happen to all those extra cycles that faster hardware is going to give us in the next hundred years. They're nearly all going to be wasted.
> I learned to program when computer power was scarce. I can remember taking all the spaces out of my Basic programs so they would fit into the memory of a 4K TRS-80. The thought of all this stupendously inefficient software burning up cycles doing the same thing over and over seems kind of gross to me. But I think my intuitions here are wrong. I'm like someone who grew up poor, and can't bear to spend money even for something important, like going to the doctor.
So I believe you are quite wrong. It is trivial to scale-up a simulation to the point where you're at a performance bottleneck. The only reason we don't scale things up today is because our computers are far too slow to handle it.
And we stopped using Gouraud shading because it is horrendously ugly. We'd still be using it still if it looked half as good as rasterization does.
You can always slow the rendering times way down by adding a lot of reflection and refraction, but those are things you can't really do in a general and physically-correct way in a polygon rasterizer.
The content pipeline is relatively simple for ray-traced content: just define the material properties of the surfaces of your scene, and the transport properties of the media your light is flowing through, and the light bounces where it bounces.
The limiting factor for real-time graphics applications is generally the cost of content creation. Real-time ray tracing could mitigate that cost.
One of the reasons VFX has moved to raytracing/pathtracing is that it copes with complexity a lot better in general (we're talking scenes with 500 million uninstanced triangles).
It also allows you to do much more accurate camera projections like spherical / fisheye (which could be useful for VR). Doing things like omni-directional spherical 3D projection in rasterisation requires tricks or warping in 2D, whereas it's pretty trivial to trace rays from the camera and do it completely accurately with raytracing.